As is well known, prospecting for minerals, hydrocarbons such as oil and gas, and other natural resources of commercial value is economically an extremely important activity. For various reasons those wishing to extract resources from below the surface of the ground or the floor of an ocean need to acquire as much information as possible about both the potential commercial worth of the natural resources in a geological formation and also any difficulties that may arise in extracting them to surface locations at which they may be used.
Techniques of logging of subterranean formations have developed for the purpose of establishing, with as much accuracy as possible, information as outlined above both before extraction activities commence and also while they are taking place.
During exploratory drilling operations, a hole is drilled from a surface location to a location underground near where the prospective resource is located. The resulting borehole may extend for several thousand or tens of thousands of meters from a surface location.
Drill pipe is typically a hollow, thick-walled, steel piping used on drilling rigs to facilitate the drilling of a borehole/wellbore. Drill pipe consists of numerous pieces, sometimes called “stands”, screwed one to another. Each stand is approximately ten meters long. Usually a stand has external male threads at one end and female threads in the internal diameter of the other end. The male threads of one stand are engageable with the female threads of another stand, thereby allowing joining of the stands together.
Normally while borehole drilling is carried out, a compound string of drillpipe stands is used in order to drive a rotatable drill bit mounted at the end of the pipe in contact with the geological formation being drilled.
As the drill bit works its way down into the ground and the borehole length increases, the drill pipe is repeatedly extended by adding new stands to its upper end. As a result long lengths of drillpipe may be inserted into boreholes as they are formed.
Broadly stated, logging involves inserting a logging tool, also known as a sonde, into a borehole or other feature penetrating a formation under investigation; and using the sonde to energize the material of the rock, etc., surrounding the borehole in some way. Such passage of the energy alters its character. The logging tool, that is capable of detecting energy, is intended then to receive emitted energy that has passed through the various components in the rock before being recorded by the logging tool.
Typically the logging tool is formed as an elongate, rigid cylinder that may be e.g. one to five meters long.
Wireline, as is well known in the art, is an armored cable that may be used for the purposes of lowering a logging tool into the borehole, or supporting the tool while it is being withdrawn upwardly along a borehole or well during logging. The logging tool is located at the end of the wireline. Logging measurements are in one known method taken by lowering the wireline including a logging tool attached as aforesaid to a prescribed depth and then raising it out of the well while operating the logging tool. Wireline is capable of electronically telemetering data from various types of logging tool from downhole to surface locations; and also of sending electronic commands to connected downhole equipment. In some situations however it is not possible or desirable to maintain the wireline connected to the logging tool following deployment of the latter.
Wireline drop-off is a conveyance system that allows for openhole data acquisition while tripping (i.e. the act of pulling the drill pipe out of the hole or replacing it in the hole). In this conveyance technique, a logging tool powered by a battery having a memory function is conveyed downhole by wireline through the drill pipe and hangs into the openhole on a no-go at the bottom of the drill pipe.
When drilling has reached total depth (the planned end of the well measured by the length of pipe required to reach the bottom), the wireline is released into the drill pipe. Typically there is a landing collar in the internal wall of the drill pipe, located near the mouth of the final (i.e., most downhole) stand, which receives a landing ring located on and protruding outwardly from the tool. The engagement of the landing ring and collar secures the tool and pipe one to another. When this engagement has occurred, the wireline is removed from the well.
The result of this sequence is that part of the logging tool protrudes beyond the end of the drill pipe and therefore is exposed in a way that permits logging of the formation. A further part of the logging tool remains inside the drill pipe and defines the described landing ring connection to the drill pipe.
To withdraw the drill pipe, the stands at the surface are unscrewed one by one from each other to separate them as the drill pipe is pulled upwardly in discrete steps. As a result the drill pipe is gradually withdrawn from the borehole. A dropped-off logging tool therefore moves towards the surface with the pipe, taking records (well logs) of the formation along the way.
Each time a drill pipe stand is to be removed from the upper end, the withdrawal operation is interrupted while unscrewing of the drill pipe takes place.
The protruding nature of the landing ring on the tool may prevent it from entering drill pipes having a small internal diameter.
Prior art wireline drop-off techniques only enable the tool to log the openhole beyond the landing ring into which the tool protrudes. This limits the length of the openhole where formation data could be acquired. It would be desirable to log openhole that is well beyond the end of the drillpipe.
Sometimes it is desirable to re-log the formation to prove the accuracy and repeatability of the measurements, a technique that is commonplace in conventional wireline logging.
Furthermore, should the drill pipe become stuck in the wellbore a common technique to free it is to rotate and reciprocate the drill pipe. Reciprocation, e.g., moving the pipe up and down, is not possible with logging tools hanging out of the end of the pipe.